1. Field of Invention
The present invention relates to a technical field of ultra-short laser amplification, and more particularly to a method for noise filtering in an optical parametric chirped-pulse amplifier and a device therefor.
2. Description of Related Arts
The seed beam in a conventional optical parametric chirped-pulse amplifier (OPCPA) is only temporally chirped. During the parametric amplification, the seed generally suffer from three kinds of noises: parametric super-fluorescence (PSF), pump distortion-induced noise (PDN) and surface-reflection-initiated pre-pulses (SRP). These noises then transform into broad pedestals or isolated side-pulses located around the intense main pulse after the amplified signal is compressed, which limit the temporal contrast of the amplified ultra-short pulse to the level of 108˜109. This contrast bottleneck arises as a combined result of the seed pulse's quality, noises from amplification and imperfections in stretcher and compressor. Contrast enhancement is always one of the most difficult problems in high peak-power laser engineering and application. Conventional technologies of pulse cleaning such as saturable absorber and cross-polarized wave generation are only applicable for cleaning the seed pulse with energy generally less than 1 mJ. There is few solution to the contrast degradation during the amplification course.
The difficulty of filtering noises that grow during amplification comes from the fact that the noises and signal usually overlap each other in all of the spatial, temporal and spectral domains. Despite the noises present different temporal structures with the main pulse after compression, there is so far no efficient temporal filtering technology with controlled switching time at femtosecond scale. The nonlinear temporal domain filtering technologies that have been used in cleaning seed pulses such as cross-polarized wave generation, are not applicable at the end of the amplifier due to the low energy conversion efficiencies of 10%-30%.